The invention relates generally to a control system for driving an alternating current (AC) motor, and more specifically relates to a variable speed control system for an induction motor. Still more specifically, the invention relates to a variable speed motor control system for controlling the vertical movement of a hoist.
The speed of a squirrel cage induction motor is proportional to the power line frequency, number of poles, and the slip of the motor. The slip of the motor is directly related to the electrical energy applied to the input power terminals of the motor. The torque of the motor is proportional to the square of the voltage applied at the input to the motor. Thus, by varying the voltage magnitude per unit of time applied at the input power terminals, the torque and the speed of the motor can be varied. In the hoist control described herein solid state thyristors are inserted between the input power source and the input power terminals of the induction motor, and voltage to the thyristors are phase controlled. The torque and speed of the motor is varied by varying the time duration of current flow per AC cycle from the source to the motor input power terminals.
A hoist is an invaluable tool for moving loads from one vertical level to another. For example, overhead industrialized trolleys include a hoist assembly for lifting the load located in one location to a safe height, then moving the trolley to a second location, and then lowering the load in place. For fragile or loosely packed material, or smoldering liquids, it is imperative to smoothly lift and lower the load without any jerking or swinging action, and to prevent even slight impact with the ground surface.
In many of the more sophisticated and costly prior hoist systems, loads were softly lifted and lowered in place. However, even in these prior systems, a jerking action would often occur upon varying the hoist speed, and frequently, upon lowering the load, the load would slip away, and, at times, would even impact with the ground. The subject invention overcomes these problems by providing a hoist control system, which controls the locking and releasing of the electro-mechanical load brake to minimize the possibility of a runaway load.
Moreover, the hoist control system of the invention herein further includes automatic quick response means to cause rapid motor reaction when the motor is switched into speed (low or high) from a zero speed condition, and thereby cause the motor to quickly gain control of the load, prior to the occurrence of minimal, if any, load slippage.
It is contemplated that the principles and the various parts of the circuitry of the subject invention are suitable and adaptable for use with other type systems, particularly those systems which require control of the speed of the motor, such as, for example, cranes, conveyors, pumps, fans etc.